The principal aim of this research project is the determination of the tertiary structure of an intact IgG immunoglobulin molecule by analyzing the X-ray diffraction pattern produced by its single crystals. An electron density map of the molecule calculated using phase angles determined by the multiple isomorphous replacement method has been interpreted by using known structures of various fragments of the intact molecule. These are the Fc fragment structure (Huber, et al), and the Fab fragment structure (Poljak et al). The Fc fragment, the Ch1 and CL domains of the Fab fragment, and the VH and VL domains of the Fab fragment were separately rotated, translated and superposed on the calculated electron density map and the superposition estimated in each case; the true orientation of each fragment in the unknown intact molecule gave the maximum superposition. Having thus determined the approximate orientation of these fragments treated as a rigid body, it is proposed to refine the structure by making changes in each domain and fitting on the calculated electron density map. This will be done using computer graphic display facility, displaying electron density map and superposing the model on it at the same time. The resulting model will be used to refine the phase angles and to calculate a better electron density map. Further refinement will be carried out by the real space refinement technique.